The present invention relates to a surface shape recognition sensor device and, more particularly, to a surface shape recognition sensor device which senses a fine three-dimensional pattern such as a human fingerprint or animal noseprint.
As application examples of a surface shape recognition sensor device for detecting the surface shape of an object to be detected, a number of fingerprint sensors for detecting a fingerprint pattern have been proposed. One is described in, e.g., “ISSCC DIGEST OF TECHNICAL PAPERS” February 1998, pp. 284–285.
In this sensor, a sensor electrode is formed in each of cells (to be referred to as sensor cells hereinafter) that are two-dimensionally arrayed on an LSI chip. An electrostatic capacitance formed between the sensor electrode and the skin surface of a finger that comes into contact with the sensor electrode via an insulating film is detected, thereby sensing the three-dimensional pattern of the skin surface. Since the value of formed capacitance changes depending on the three-dimensional pattern of the skin surface, the three-dimensional pattern of the skin surface can be sensed by detecting the capacitance difference.
A surface shape recognition sensor device having a means for individually adjusting the detection sensitivities of a plurality of sensor circuits can be supposed.
FIG. 10 shows a use state of such a surface shape recognition sensor device having a detection sensitivity adjusting function. This surface shape recognition sensor device is constructed by a number of sensor cells adjacent to each other. Typically, the surface shape recognition sensor device is formed from a number of sensor cells 11 two-dimensionally arranged (in an array or grid shape).
An object to be detected, such as a finger 13, is brought into contact with a sensor surface 12 of a surface shape recognition sensor device 10. A surface (three-dimensional shape of a fingerprint) 14 to be detected is individually detected by each sensor cell 11, and two-dimensional data representing the surface shape of the object to be detected is output.
FIG. 11 shows the entire arrangement of the conventional surface shape recognition sensor device.
The sensor cells 11 are arranged in a two-dimensional matrix with p rows×q columns. The sensor cells 11 are connected to word lines WL1 to WLp for selectively controlling the sensor cells 11 and data lines DL1 to DLq for propagating the outputs from the sensor cells 11 so as to form a grid shape.
A decoder 20 and A/D conversion circuit 30 are arranged in the periphery of the sensor cells 11. The decoder 20 controls the word lines in accordance with a received address signal ADR. The A/D conversion circuit 30 converts analog signals received from the data lines into a digital signal and outputs it.
In this way, in the surface shape recognition sensor device 10, a plurality of sensor cells 11 are sequentially selected by the decoder 20, and the analog outputs from the sensor cells 11 are converted into digital outputs by the A/D conversion circuit 30. The sensor cells 11 share the A/D conversion circuit 30. With this arrangement, any increase in circuit scale is suppressed, and the analog outputs from a number of sensor cells 11 are efficiently converted into digital outputs.
The surface shape recognition sensor device 10 has a function of adjusting (to be referred to as calibration hereinafter) the sensitivity of each sensor cell to uniform a variation in characteristic, i.e., detection sensitivity between the sensor cells 11 due to the manufacturing process.
In the conventional surface shape recognition sensor device 10, calibration mode signal lines CL1 to CLp are connected to the plurality of sensor cells 11, like the word lines WL1 to WLp.
When a calibration mode signal line is activated, a calibration control circuit 40 sequentially selects a plurality of sensor cells and sequentially calibrates the selected sensor cells.
FIG. 12 shows a conventional sensor cell. The sensor cell 11 is formed from a detection element 1, a sensor circuit 2, a calibration circuit 3, and a selection circuit 4.
The detection element 1 converts a surface shape into an electric amount. The sensor circuit 2 measures the electric amount of the detection element, which changes depending on the surface shape, causes an internal voltage-time conversion circuit (to be referred to as a VT conversion circuit hereinafter) 21 to convert the electric amount into a time signal having a pulse width corresponding to the electric amount, and outputs the time signal as an output signal 2A.
The calibration circuit 3 individually executes adjustment (sensitivity adjustment) of the detection sensitivity of the sensor circuit 2 in each sensor cell 11. The selection circuit 4 sets the sensor cell 11 in an operative state on the basis of the active state of the word line WL or the calibration mode signal from the calibration mode signal line CL.
To calibrate the detection sensitivity of the sensor circuit 2 in each sensor cell 11, a reference sample without any three-dimensional pattern is detected by the sensor, or detection is performed without placing anything on the sensor surface, thereby causing the respective sensor cells 11 to detect the same measurement value.
The output signal 2A from each sensor cell 11 is input to the calibration circuit 3 of that sensor cell. The calibration circuit 3 is formed from a load circuit 31, a counter circuit (n-bit) 32, and a time signal comparison circuit 33.
The time signal comparison circuit 33 compares the output signal 2A with a reference pulse signal 3A having a pulse width corresponding to a desired detection sensitivity. A comparison result having a pulse width corresponding to the difference between them is input to the counter circuit (n-bit) 32 as a counter input signal 3B.
The counter circuit 32 executes new counter operation on the basis of the counter input signal 3B. The count data of the counter circuit 32 is sequentially updated. Connection of the sensor circuit 2 to n load elements Z1 to Zn arranged in the load circuit 31 is controlled on the basis of the count data so that the detection sensitivity of the sensor circuit 2 is adjusted.
As the load elements Z1 to Zn in the load circuit 31, load elements that can be controlled to active and inactive states may be used. FIGS. 13A and 13B show implementation examples of load elements that can be controlled to active and inactive states. FIG. 13A shows capacitive load elements. FIG. 13B shows resistive load elements. When an electrostatic capacitance formed between the finger surface 14 and a sensor electrode 1B is used as the electric amount, one of the load elements shown in FIG. 13A is used. When a contact resistance formed between the finger surface 14 and the sensor electrode 1B is used as the electric amount, one of the load elements shown in FIG. 13B is used.
This operation is repeated a number of times corresponding to the number of adjustment stages, e.g., 2n−1 times except a state wherein all load elements Z are unselected, thereby adjusting the detection sensitivity of each sensor circuit 2 and uniforming the performance of the sensor cells.
As shown in FIG. 12, the detection element 1 is implemented by the sensor electrode 1B formed on an insulating layer 16 and covered with a passivation film 15. As the electric amount, an electrostatic capacitance Cf formed between the finger surface 14 and the sensor electrode 1B is used.
The sensor circuit 2 is formed from a Pch MOSFET Q1, an Nch MOSFET Q2, an constant current source I, and the VT conversion circuit 21. Reference symbol Cp0 denotes a parasitic capacitance.
FIG. 14 shows the detection operation of a sensor cell.
Before time T1, a sensor circuit control signal {overscore (PRE)} is controlled to a power supply voltage VDD to turn off the Pch MOSFET Q1. A sensor circuit control signal RE is controlled to 0 V to turn off the Nch MOSFET Q2. A node N1 is set at 0 V.
At the time T1, the signal {overscore (PRE)} is controlled to 0 V to turn on the Pch MOSFET Q1. The potential of the node N1 rises up to VDD. At time T2, the signals {overscore (PRE)} and RE are controlled to VDD to turn off the Pch MOSFET Q1 and turn on the Nch MOSFET Q2. With this operation, charges accumulated in the electrostatic capacitance Cf are removed.
Hence, the potential of the node N1 gradually drops at a rate depending on the electrostatic capacitance Cf. At time T3 after the elapse of a predetermined time Δt from the time T2, the signal RE is controlled to 0 V to turn off the Nch MOSFET Q2. At the node N1, a potential VDD−ΔV corresponding to the electrostatic capacitance Cf is maintained. This potential is output to the VT conversion circuit 21.
The VT conversion circuit 21 has a constant current source IVT, a capacitance CL, and a threshold value circuit 22.
In the VT conversion circuit 21, the constant current source IVT operates in accordance with the potential of the node N1 to charge the capacitance CL. When the potential of the capacitance CL exceeds a predetermined threshold value VTH, the threshold value circuit 22 inverts its output, i.e., the output signal 2A.
The output signal 2A is inverted after the elapse of a time corresponding to the potential of the node N1 from the start of charge accumulation in the empty capacitance CL. In sensing operation of detecting the surface shape of an object to be detected, this time length is measured, thereby detecting the three-dimensional pattern of the skin surface.
In calibration operation of adjusting the detection sensitivity of the sensor circuit 2, the time signal comparison circuit 33 operates on the basis of a calibration signal CAL from the calibration control circuit 40, and calibration by the calibration circuit 3 is executed. At this time, count data in the counter circuit 32 is set to the initial set value in advance to set all load elements in an inactive state. The output signal 2A from the VT conversion circuit 21 is set to the initial set value at the start of individual detection operation. The reference pulse signal 3A having a pulse width corresponding to a desired detection sensitivity is supplied to the sensor cell 11. In synchronism with this, the sensor cells 11 sequentially execute detection operation.
With this operation, the output signal 2A is obtained from the sensor circuit 2. As shown in FIG. 15, the time signal comparison circuit 33 compares the output signal 2A with the reference pulse signal 3A. For example, the output signal 2A and reference pulse signal 3A are ANDed to generate the counter input signal 3B.
When the electrostatic capacitance Cf is constant, a delay time ts from the leading edge of the reference pulse signal 3A to the leading edge of the output signal 2A changes in accordance with the load in the load circuit 31.
When the output signal 2A has changed before the time determined by tr (ts<tr) (times T11 and T12), the counter input signal 3B is input to the counter circuit 32 for every detection operation. Hence, the counter circuit 32 increments the count data, and the load of the load circuit 31 sequentially increases.
When the output signal 2A has changed after the time determined by tr (ts≧tr) (time T13), the detection sensitivity of the sensor circuit 2 equals the desired sensitivity. The counter input signal 3B is not output anymore, and the selection state of the load elements at that time is held by the counter circuit 32.
As the load value of each load element, e.g., Zk=Z·2k−1 (k is a natural number) is set. Every time the count data is incremented, the value of the load circuit 31 is increased by Z. The detection sensitivity can be adjusted in Z increments.
Such calibration operation is individually executed for the respective sensor cells 11 to adjust them to an appropriate detection sensitivity. Even when the characteristic, i.e., detection sensitivity of the sensor circuit 2 changes between the sensor cells due to the process variation, the sensor cells can have uniform detection performance.
However, in such a conventional surface shape recognition sensor device, in adjusting the characteristics of the sensor circuits in the sensor cells, a desired sensor cell is selected by the calibration control circuit. This undesirably increases the circuit scale of the surface shape recognition sensor device. Especially, since the calibration control system is added to a control system for controlling normal surface shape detection operation, the circuit arrangement and control become complex. As a result, the chip area increases, the manufacturing yield degrades, and the design time is prolonged. This increases the manufacturing cost as a whole.
The present invention has been made to solve this problem, and has as its object to provide a surface shape recognition sensor device capable of calibrating the detection sensitivity of each sensor cell with a smaller circuit arrangement.